function validarPedido(){
	var arrStock = document.getElementsByTagName("select");
	for(var i=0; i<arrStock.length-1; i++){
		for(var j=i+1; j<arrStock.length; j++){
			if(arrStock[i].options[arrStock[i].selectedIndex].value == arrStock[j].options[arrStock[j].selectedIndex].value){
				alert("Ha seleccionado la misma unidad para dos o mas articulos.");
				return false;
			}
		}
	}
	return validar();
}